The invention relates to an arrangement for the generation of an FSK-modulated optical signal with two polarization states that are orthogonal to one another from an FSK-modulated optical transmitted signal.
In an optical transmission system with superheterodyne reception, it is necessary that the local oscillator signal have the same polarization state as the received signal in order to obtain a maximum signal amplitude of the intermediate-frequency signal. Since the birefringence of a conventional monomode fiber is influenced by thermal as well as mechanical disturbances, the polarization state of the received signal fluctuates, which leads to amplitude fluctuation of the intermediate-frequency signal. In the event that the local oscillator signal and the received signal are polarized in orthogonal fashion to one another, the intermediate-frequency signal disappears completely.
A reception independent of polarization is possible if the optical signal supplied to the receiver has polarization states which are orthogonal to each other. Such a signal can be generated from the transmitted signal, e.g. via a polarization switch controlled by the data signal to be transmitted (see German patent application P 38 33 273.6 or German patent application P 38 33 274.4, both incorporated herein by reference).
A method of this type is suitable for all known digital methods of modulation, i.e. ASK-, DPSK- and FSK-modulation. However, additional electronic units are necessary in order to control the polarization switch. Furthermore, for each optical transmitter a switch with corresponding control electronics must be provided in a multichannel system.
In an optical transmission system for FSK-modulated optical transmitted signals a polarization change can be caused by this signal itself. For this, it must go through a suitable birefringent medium first before the actual transmission via the fiber. The literature suggests the use of a polarization-maintaining fiber of suitable length (see Electron. Lett. 25, 1989, p. 4-5, incorporated herein by reference). If one couples into this fiber the FSK-modulated signal under 45 degrees relative to the principal/main axes of the polarization maintaining fiber, a signal is obtained at the fiber output with polarization states which are orthogonal to each other and which are changed over synchronously to the modulation. Given a frequency deviation of e.g. .DELTA.f=1 GHz compared to the basic frequency f.sub.o of the modulated optical signal and a fiber birefringence of B=5.10.sup.-4, a fiber of approximately 300 m length must be used.
Another possibility to generate an FSK-signal with orthogonal polarization states is suggested in the literature proposing to use a Mach-Zehnder-interferometer (see IEEE J. Lightwave Technol. LT6 (1988), p. 1537-1548, incorporated herein by reference). In order to generate orthogonal polarization states given a frequency deviation .DELTA.f of the FSK-modulated transmitted signal at the interferometer output, the optical path difference .DELTA.l between the arms of the interferometer must be equal to c/2 .DELTA.f, whereby c is the light velocity. This interferometer requires two polarizing beam splitters and its adjustment involves great expense.